1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), a method for determining a gray level in dynamic capacitance compensation (DCC) on the LCD, and a method for correcting a gamma of the LCD, and more particularly, to an LCD to improve the quality of moving and still images, a method for determining a gray level in DCC for the LCD to improve moving picture quality, and a method for correcting a gamma of the LCD to improve still picture quality.
2. Description of the Related Art
Cathode ray tubes (CRTs) are being gradually replaced by flat panel displays such as LCDs, plasma display panels (PDPs), and organic electroluminescent displays (OELDs). LCDs are popular because they are lightweight and thin.
LCDs include an upper glass substrate on which a common electrode and a color filter are formed, a lower glass substrate on which a thin film transistor (TFT) and a pixel electrode are formed, and an anisotropic dielectric constant liquid crystal filled between the upper and lower glass substrates. An electric field intensity in the liquid crystal is controlled by a potential that is independently applied to the pixel electrode and the common electrode. The electric field alters the molecular configuration of the liquid crystal and thus controls the amount of light transmitted through the substrates to display a desired image. For example, LCDs using a TFT as a switching device are most commonly used, which are referred to as TFT-LCDs.
In a typical LCD, a gray scale voltage to be applied to a pixel electrode is determined according to a gamma value. FIG. 1 is a circuit diagram illustrating a method for adjusting a gamma value on a conventional LCD. Referring to FIG. 1, once the gamma value is determined, each voltage VO<0> through VO<255> that is applied to a corresponding node between a row of serially connected resistors R0 through R255 is used as a gray scale voltage corresponding to the gamma value by adjusting the ratio of resistances of resistors in the row. Correcting the gamma according to the type of liquid crystal in an LCD or environmental illumination makes it possible to control the brightness across the entire screen, which improves still picture quality. However, in conventional LCDs, a resistance value must be adjusted by replacing resistors in each row or using a variable resistor in order to correct a gamma value. Thus, it is difficult to improve the quality of still images.
When a gray scale voltage is applied to a pixel electrode in an LCD, a liquid crystal material takes time to respond to the gray scale voltage. Thus, due to the time delay required to display the desired image, it is difficult to display a moving image.
Dynamic capacitance compensation (DCC) is a technique that has been developed to improve the response speed of the LCD. The DCC technique minimizes the time delay by applying a gray scale voltage that is greater than the original gray scale voltage to a pixel electrode.
FIG. 2 is a graph of an optimal liquid crystal response curve applicable for applying a DCC technique on a conventional LCD. Referring to FIG. 2, in the conventional DCC technique, after comparing a gray scale voltage Gk- 1 for a previous frame with a gray scale voltage Gk for a current frame, each of gray scale voltages Gbst1, Gbst2, and Gbst3 that is greater than the difference between the gray scale voltages Gk- 1 and Gk, plus the gray scale voltage Gk- 1 for the previous frame, is applied during a single frame period (e.g., 1/60 sec in the case of a 60 Hz frame rate), shown as 1 frame in FIG. 2, and the original gray scale voltage Gk is applied after the frame period.
An optimal liquid crystal response curve Res2 is then determined from among the measured liquid crystal response curves Res1, Res2, and Res3. The gray scale voltage corresponding to the optimal response curve Res2 and the gray scale voltages for the previous and current frames are mapped to gray scale levels that are stored in a look-up table (LUT). Since the optimal liquid crystal response curve Res2 is determined by observer's judgment and not a calculation, the optimal liquid crystal response curve Res2 suffers from an inter-observer error or a discrepancy in each measurement, making it difficult to objectively apply a DCC technique. Further, since a separate DCC LUT is required for each corrected gamma value, it is difficult to improve both moving and still picture quality.